Rostoker’s colliding beam reactor works
by creating two compact toroids and then
accelerating them at the supersonic speed
of 250 kilometers per second into a head-on collision. The toroids merge into a
toroid known as a field-reversed configuration (FRC), converting their kinetic energy
into heat. Extra heating is provided by ion
beams. The Tri Alpha team has also developed tricks to lengthen the lifetime of the
FRC, reporting lifetimes of up to 4 milliseconds in a 2012 paper in Physical Review
Letters. Unlike Tri Alpha’s rivals, the device
does not compress the plasma but instead
relies on high temperature and long confinement to spark fusion.

Tri Alpha’s current machine, called the C-2,
is the length of a tennis court, according to

Glen Wurden, magnetized plasma team
leader in the Plasma Physics Group
at Los Alamos National Laboratory
(LANL) in New Mexico, who has
visited the facility. “It’s a very
beautiful machine,” he says.

THE VISIONARY Robert
Bussard started his career
at LANL designing nuclear-powered rocket engines, including the Bussard ramjet,
which uses a magnetic field
to scoop up interstellar hydrogen to fuel its fusion engine. His
ramjet is a common propulsion
system in science fiction novels, if
not in real spacecraft. Later, he joined
forces with Bruno Coppi, a fusion researcher from the Massachusetts Institute
of Technology, to develop a novel fusion reactor they called the Riggatron—after the Riggs
National Bank, an early backer. Their biggest
supporter was Bob Guccione, the flamboyant
publisher of magazines including Penthouse
and Omni. But just as they were planning
a public flotation, Guccione balked and the
project collapsed.

Bussard quickly recovered from that setback and by the mid-1980s had come up with
another promising fusion design: the polywell. The polywell is a refinement of another
device known as a fusor, perhaps the simplest of fusion reactor designs. A fusor has
two usually spherical electrodes made of wire
mesh, one inside the other. When the setup
is placed in a vacuum chamber filled with fusion fuel and a large voltage is applied across
the electrodes, the electric field accelerates
ions inward, toward the inner mesh. In theory, the ions fly right through the holes and
continue on to the center where they collide
with other ions and fuse. The problem is that
too many ions hit the inner electrode and are
absorbed, cutting the device’s efficiency and
putting ignition out of reach.

Bussard’s idea was to replace the inner
electrode with something that was harder
to hit: a virtual electrode. The polywell is
made up of a number of ring-shaped electromagnets, usually arranged to form a cube.
When current is passed through the magnets, they create a field that has a null point
in the center of the cube, which traps any
charged particles. An electron gun fires
electrons through the middle of the rings
and they become trapped by the field. Once
enough are in place, the electrons act as an
electrode, exerting a strong pull on positive ions. Atoms of fuel are puffed in at the
corners, become ionized, accelerate into the
center, and, with luck, collide with other
ions and fuse.

Bussard set up the Energy/Matter Conver-sion Corp. (EMC2) in 1985 to research poly-wells. He won funding from the Departmentof Defense and later from the U.S. Navy. ButJaeyoung Park, who now leads EMC2, saysBussard was “conceptually very good, but notan experimentalist.” In 2005, the Navy cut thecompany’s funding, and Bussard embarkedon a publicity campaign, highlighted by atalk at Google headquarters titled “ShouldGoogle Go Nuclear?”In August 2007, the Navy restored its fund-ing with $1.8 million and a new team was as-sembled at EMC2, including Richard Nebeland Park, both on leave from LANL. Then, inOctober, Bussard died of multiple myelomaat the age of 79. Nebel and Park were left, soto speak, holding the baby.

The polywell’s big problem is confinement:Particles leak out through gaps in the mag-netic field. In experiments carried out lastOctober, EMC2 used improved electron gunsto build up a high pressure of electrons in thecenter and showed that confinement was sig-nificantly improved. “We’ve taken a big stepforward,” Park says. “We were behind Tri Al-pha, now we’re competing directly.”Despite this success, the Navy told EMC2that it would be stopping its funding laterthis year. So Park is back on the money trail,seeking $30 million for a 3-year program toput polywell to the test.

SCALING UP Which of the dark horses is in
the lead? It’s hard to say. And there are other
contenders: companies with projects ranging from a compact version of a tokamak—
a mainstream fusion device—to muon-catalyzed fusion, an exotic approach
that relies on this heavy cousin of the
electron to drag pairs of nuclei close
enough together to fuse. Most have
produced promising laboratory-scale devices; all have persuaded
some venture capital companies
and wealthy individuals that
their investments could, one day,
spawn an immensely lucrative
industry.

Mainstream fusion scientistshave mixed opinions of the break-away efforts. “The center line is theyare long shots,” Princeton’s Pragersays. “It would be great if they succeed,The next stage—building a demonstra-tion reactor that can get close to breakevenor even reach ignition—will require a wholenew level of funding: not millions of dollarsbut hundreds of millions. But the startupsare undaunted. “There are a lot of potentialinvestors out there, a lot of wealthy individu-als prepared to take on a big challenge,” saysDavid Kingham, CEO of Tokamak Energy, aBritish venture working on a compact toka-mak. “Private money can sometimes achievethings that public money can’t.”Park thinks the chase inspires people. “It’sa fascinating subject, it makes people’s heartsbeat quicker,” he says. But will any of theseschemes work? “We will never know untilsomeone really cracks it,” Park says. But headds: “I like our chances.” ■